Simulation of the UQ Gatton natural draft dry cooling tower

Accepted Manuscript

Simulation of the UQ Gatton natural draft dry cooling tower

Xiaoxiao Li, Zhiqiang Guan, Hal Gurgenci, Yuanshen Lu, Suoying He

PII: S1359-4311(16)30333-7

DOI: http://dx.doi.org/10.1016/j.applthermaleng.2016.03.041

Reference: ATE 7909

To appear in: Applied Thermal Engineering

Received Date: 23 November 2015

Revised Date: 27 February 2016

Accepted Date: 9 March 2016

Please cite this article as: X. Li, Z. Guan, H. Gurgenci, Y. Lu, S. He, Simulation of the UQ Gatton natural draft dry

cooling tower, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.03.041

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17th IAHR International Conference on Cooling Tower and Heat Exchanger, 7-11 September 2015, Gold Coast,

Queensland, Australia

Simulation of the UQ Gatton natural draft dry cooling tower

Xiaoxiao Li1 *, Zhiqiang Guan

1, Hal Gurgenci

1, Yuanshen Lu

1, Suoying He

2

1 School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Australia

2School of Energy Source and Power Engineering, Shandong University, Jinan, China

Highlights:

1. The full scale 1-D and 3-D simulation models of a 20m NDDCT was developed.

2. The ambient temperature and hot water inlet temperature effect had been investigated

3. The crosswind effect on the small NDDCT was analysed and discussed

ABSTRACT: A Natural draft dry cooling tower (NDDCT) is a cost-effective cooling

technology which can be utilized in most of the small renewable power plants. While a

number of numerical studies have been done on NDDCTs in recent decades, experimental

studies on full scale small cooling tower are very few. To fill this gap, Queensland

Geothermal Energy Centre of Excellence (QGECE) has built a 20 m NDDCT. In this study, a

1-D analytical and a 3-D CFD models of this cooling tower were developed and the cooling

performance was investigated at different ambient temperatures, inlet water temperatures and

crosswind speeds. The results show that NDDCT in such a size is capable for a 2~3 MW CST

power plant. The cooling performance of the NDDCT decreases with the increase in the

ambient temperature and the decrease in the inlet water temperature. In terms of the

crosswind, the heat rejection ratio decreases with the increase of the crosswind velocity at

low crosswind speeds. However, when the crosswind speed becomes large enough, the heat

dumped at the bottom of the tower can compensate some losses in cooling capacity caused by

crosswind. The results found in the present study give reference for future tests.

Key words: Natural draft dry cooling tower (NDDCT), Cooling performance, CFD modelling

* Corresponding author. E-mail address: [email protected] (X. Li)

Nomenclature

A Area (m2)

C Inertial resistance factor

Cp Specific heat (J Kg-1

K-1

)

CD Drag coefficient

d Diameter (m)

h Empirical heat transfer (W m-2

K-1

)

H Height, elevation (m)

K Flow resistance

L length (m)

m Mass flow rate (kg/s)

n Number

P Pressure (Pa)

Q Heat transfer rate (W)

q Heat flux (W m-2

)

Re Reynolds Number

T Temperature (°C)

v Velocity scalar (m s-1

)

Greek letters

α Permeability

δ Enthalpy of air (J) σ Compact area ratio of the heat exchanger

ε Turbulent kinetic energy dissipation (m2

s-3

)

η Efficiency

κ Turbulent kinetic energy (m2 s

-2)

ρ, Density, mean density (kg m-3

)

μ Viscosity (kg m-1

s-1

)

Vectors

F

Force

v

Velocity

kji

,, Unit vectors of x-, y-, z- direction in

Cartesian coordinate system

Subscripts

a, w Air side, water side

cw Crosswind

e Effective

fr front area

he Heat exchanger

i, o Inside or inlet, outside or outlet

t Tube

u Over all

0 Reference value

ts Tower support

ctc Heat exchanger compact

cte Heat exchanger expansion

cto Tower outlet

1,2,3,4,5 Different location of cooling tower

1. INTRODUCTION

Natural Draft Dry Cooling Towers (NDDCTs) have been successfully used in traditional

thermal power plants for several decades. Towers used in such plants are typically more than

100 m in height [1, 2]. Geothermal power plants and concentrated solar thermal (CST) power

plants are promising renewable power generation systems. Compared with conventional

thermal power plants, these plants are likely to be smaller and usually located in water

deficient areas [3]. Short-NDDCTs, which can effectively discharge heat and do not consume

any water and electricity, are likely to be a feasible cooling technology for these kinds of

power plants [4, 5] . Past studies have identified the vulnerability of short natural draft towers

to ambient influences, especially the crosswind and the hot ambient temperature [6-9]. This

paper describes the simulated performance of a 20m tall experimental NDDCT constructed at

the University of Queensland.

Kroger et al. [1] summarized the past researches on air-cooled heat exchangers and the

fluids mechanics and proposed a useful 1-D analytical model to predict the cooling efficiency

of NDDCTs in the absence of crosswind. Their 1-D model combines the energy balance

equations and the air flow draft equations and have been validated against the industrial data

[1]. When these two groups of equations are both satisfied, the cooling performance of the

NDDCT can be calculated.

The influence of the crosswind has received significant attention in the last few

decades[10-12]. Su et.al [13] simulated the fluid flow and the temperature distribution of the

dry cooling tower under the crosswind. The results without crosswind have been compared

against those obtained with crosswind, at the speeds of 5 and 10 m/s. Based on the simulation

result, the authors explained how the crosswind affects the cooling performance of the tower.

Al-Waked and Benhia [6] analysed the performance of a NDDCT under crosswind using a

three-dimensional computational model. The effectiveness of the windbreak walls has also

been studied in this paper. The authors proposed the ratio of the amount heat at crosswind

condition to the maximum amount of rejected heat which occurs in the windless condition to

better express the effect of the crosswind. According to their simulation results, the crosswind

velocity has a big effect on the cooling performance of the NDDCT. The cooling

performance drops by 30% when the wind velocity exceeds 10 m.s-1

. Lu et.al [14, 15]

investigated the crosswind effect on small sized NDDCTs. They examined the simulated

performance of a 15 m NDDCT with horizontally-arranged heat exchangers. Different

velocities and different angles of the wind were discussed. Zhao et al [16, 17] studied the

cooling performance of a NDDCT with vertical delta radiators under the constant heat load.

With constant heat load and uniform water inlet temperature, the cooling performance of each

heat exchanger sector was analysed under crosswind impact. The authors claimed that with

increasing crosswind velocity, the cooling performance of NDDCT under constant heat load

deteriorates sharply at low velocity, but varies slightly at high velocity.

QGECE has been developing the small size NDDCT for CST and geothermal power

generation systems [8, 14, 15, 18]. This research is necessary because the existing NDDCT

design is optimised for large stream power plant, and is not optimal for a CST plant. A 20m

high NDDCT designed for CST power plant is commissioned and will be tested for the future

application as a part of CST energy system. Based on the above literature review, for short

NDDCTs (less than 30m height), their cooling performance is more sensitive to the ambient

conditions. Therefore, an accurate simulation of the ambient influence on a small sized

NDDCT is a very important prerequisite for the future test on the same NDDCT. In this paper,

the results of the 1-D and 3-D models of the Gatton NDDCT are presented. The objectives of

this work are as follows: (1) to study the influence of the ambient temperature and crosswind

on this specific small NDDCT. (2) to prepare the future test on the same NDDCT.

2. 1-D SIMULATIONS

The simulated cooling tower is of hyperbolic shape and is 20m high with the base

diameter of 12 m. Fig. 1 presents the configuration of the tower. The 18 heat exchanger

bundles are located horizontally above the inlet cross section of the tower.

Figure 1: Gatton cooling tower configuration

To calculate the performance of the cooling tower, the energy balance equations and draft

equations must be satisfied simultaneously. The assumptions of the 1D model are as

following: 1) The cooling tower is operated in the stable condition. 2). The flow resistances

other than tower support, the heat exchanger and the tower outlet were neglected. 3) The heat

transfer process only happens in the air-cooled heat exchanger. As presented in Fig. 1, the

pressure drops between positions 1 and 5 include: the tower support resistance (Kts), the heat

exchanger compact resistance (Kctc), the heat exchanger expansion resistance (Kcte), the heat

exchanger bundle resistance (Khe) and the tower outlet resistance (Kto).

Using these five coefficients, the draft equation of the cooling tower can be expressed as

[1]

(1)

The definitions of all terms can be found in the Nomenclature table. The tower support loss

coefficient based on the conditions at the tower support is given by Kroge [1]

(2)

The heat exchanger contraction and expansion loss coefficients can be calculated using the

Eqs. (3) and (4) respectively [1].

(3)

(4)

For towers with cylindrical outlets, the loss coefficient is given by [1]

(5)

where

(6)

The logarithmic mean temperature difference method was selected to calculate heat

transfer process of the heat exchanger [19]. The heat transferred in the air-cooled heat

exchanger can be presented by Eqs. (7) or (8).

(7)

or

(8)

For NDDCTs, the air-cooled heat exchanger is one of the most important part and in an

air-cooled heat exchanger, the air side loss contributes the most heat transfer resistance [19].

To simplify the calculation, this paper correlated the heat transfer coefficient (UA) and the

loss coefficient (Khe) as functions of Reynold number of the air (Reair). The software Aspen

Exchanger Design and Rating was used to get the correlations of Reair with UA and Khe. The

simulation results have been compared with the data provided by the heat exchanger

manufacturer. The deviation of the simulation is less than 3%.

For this particular heat exchanger configuration in the Gatton cooling tower, Eq. (9) and

(10) express the correlations of Reair and UA and Khe., respectively,

(9)

(10)

Based on Eqs. (1) to (10), the 1-D model of the NDDCT can be built and a solution can be

obtained through a simple iterative procedure. Fig.2 is the flow chart of the iteration process,

which is described as the following:

1) Input the tower configuration parameters, the heat exchanger parameter, the water input

conditions and the ambient conditions;

2) Assume the outlet temperature of the heat exchanger Ta4 equal to the water inlet

temperature Twi;

3) Calculate the air mass flow rate ma using the draft equations: Eqs. (1) to (6) and (10);

4) Use the energy balance equation: Eq. (8) to get the outlet temperature of the water and

the air enthalpy change Q1;

5) Use heat exchanger equation Eqs. (7) and (9) to get the heat flux of the air-cooled heat

exchanger Q2;

6) Take n as 104, f │Q1-Q2│> n, set Ta4=Ta4-Tstep, then repeat steps 3 to 5;

7) Output the results when │Q1-Q2│< n;

Figure 2: Flow chart of the iteration process of the 1D model

3. 3-D CFD MODELLING

A one-dimensional model is not capable to simulate the crosswind effect. To investigate

the crosswind effect on the cooling tower performance, this study used ANSYS Fluent to

build a 3-D CFD model of the Gatton cooling tower. The cooling performance under

different crosswind conditions was modelled.

3.1 Mesh and geometry

The geometry of the cooling tower in this CFD model is the same as the physical size of

the Gatton cooling tower. In this simulation, the cooling tower has been divided into 3 parts:

the tower support, the heat exchanger and the main tower. In order to get an accurate result

and better compare with the future experiment, the geometry of the heat exchanger is set as

same with the experimental cooling tower. So that the single heat exchanger performance can

be calculated and compared with experimental result later. A 90 m high cylinder with a

diameter of 144 m is set as the computational domain area. Fig. 3 shows the geometry of the

3-D model.

About 1, 320, 000 unstructured prism cells are used in the model. The grid-independence

test has been done. The deviation of the results was less than 1% when the quantity of the

cells is over 1, 300, 000. The ANSYS meshing high smoothing and slow transition curvature

size function are adopted to generate the mesh. The element size of the tower and heat

exchanger is set to be 0.3m while the element size of the computation domain area is 0.5 m.

Figure 3: Geometry diagram of the 3-D model

3.2 Boundary condition

The bottom face of the computation domain and tower surface are defined as the zero heat

flux and non-slip wall with the standard wall function. In windless condition, the side face of

the cylinder is defined as the pressure inlet boundary and the pressure outlet boundary

condition is applied on the top face of the cylinder. In the crosswind condition, a velocity

inlet boundary condition is applied at the side face of the cylinder, with the velocity profile

define as:

(11)

The temperatures and pressures at both the inlet and outlet boundaries are set same as the

ambient one. Because of the low-turbulence level of advection natural wind, the impact of the

turbulence intensity and viscosity ratio was very little at the computation domain boundaries

[24]. They can be set as 0.1% and 0.1, respectively.

The heat exchanger bundles in the cooling tower are modelled by the radiator model with a

porous zone. The internal structure of the air cooled heat exchangers are simplified as a

porous media zone which represents all the air flow resistances within the heat exchangers.

Acco d ng to FLUENT us ’s gu d , the heat transfer process in the radiator can be expressed

by Eq. (12).

o (12)

The pressure drop of the system heat exchanger bundles is modelled by porous zone

through adding a momentum source term into the corresponding momentum equation. As

presented in Eq. (13), the source term is composed by two parts: a viscous loss term and an

inertial loss term.

(13)

The parameters in Eqs (12) and (13) can be correlated by calculating the 1-D model under

different conditions.

3.3 Governing Equations

According to the previous research [16-18, 20-23], the realizable k-ε model has been

selected in this study. The general term of the governing equations can be expressed as:

(14)

Table. 1 presented the expression of the three parameters in the above equation.

Table 1: The 3-D governing equation parameter from Eq. (14)

Continuity 1 0 0

x momentum U

y momentum V

z momentum W

Energy T

Turbulent energy k

Energy dissipation ε

where

;

t n

; Pr=0.74; Prt=0.85

4. RESULTS AND DISCUSSIONS

4.1 1-D simulation result

In the present 1-D simulation, the water mass flow rate is fixed at 14.8 kg.s-¹. The

temperature of the hot water inlet is ranged from 53.31 ℃ to 83.31 ℃ while the ambient

temperature varies from 15 ℃ to 45 ℃. Figs. 4 and 5 presents the variations in the air mass

flow rate and the heat rejection rate of the cooling tower.

Figure 4 The air mass flow rate of the Gatton NDDCT at different ambient temperature

15 20 25 30 35 40 4530

40

50

60

70

80

90

100

110

120

130

ma

(kg

.s-1

)

T(Co)

Twi

=53.31Co

Twi

=63.31Co

Twi

=73.31Co

Twi

=83.31Co

Figure 5 The heat rejection rate of the Gatton NDDCT at different ambient temperature

Figs. 4 and 5 show that, both the heat rejection rate and the air mass flow rate decrease as

the ambient temperature increases or the hot water inlet temperature decreases. Within the

ambient temperature changes from 15 ℃ to 45 ℃., the air mass flow rate decreases by 66%,

49%, 36% and 27%, when the hot water temperatures are 53.31 ℃, 63.31 ℃ 73.31 ℃ and

83.31 ℃, respectively. These drops in heat rejection are heat rejection rate are 87%, 71%,

58% and 48%, respectively. It can be seen in Figs. 4 and 5, the cooling tower performance is

more sensitive when the hot water temperature is close to the ambient temperature.

Eq. (15) is the simplified draft equation of the cooling tower. The right side of the equation

presents the draft provided by the cooling tower while the left side is the air flow resistance in

the cooling tower. In a NDDCT, the air flow in the cooling tower is mainly determined by the

air density difference, the draft height and the pressure loss of the heat exchanger. For a given

cooling tower, the main influencing factor of the air flow is the density difference between

the hot air inside the tower and the cool air outside the tower.

(15)

Based on the physical property of the air, the larger the temperature difference between hot

and cool air, the bigger the air density difference exists. The rise in ambient temperature and

the decrease in hot water temperature both decrease the temperature difference between the

inside and outside air of the tower. Thus, the air flow rate passing through the tower was

adversely influenced. The cooling performance of NDDCTs relies mainly on the convective

heat transfer created by the natural draft effect and thus not as effective as that in wet cooling

towers. So the cooing performance is particularly reduced when the ambient air is hot.

For the CST plant using the super-critical CO2 power cycle proposed by the University of

Queensland, the condensing temperature is around 70 ℃. When the ambient temperature

varies from 15 ℃ to 40 ℃, the heat rejection rate of the cooling tower changes

correspondingly from 3.1 MW to 1.9 MW. Considering the higher thermal efficiency of the

15 20 25 30 35 40 4530

40

50

60

70

80

90

100

110

120

130

ma

(kg

.s-1

)

T(Co)

Twi

=53.31Co

Twi

=63.31Co

Twi

=73.31Co

Twi

=83.31Co

supercritical CO2 cycle and hence the relatively smaller cooling requirement, this small size

NDDCT is suitable for the CST power plant with 2~3MW nominal net power.

4.2 3-D simulation result

The cooling performance of Gatton tower was simulated using the 3-D model described in

Section 3. The simulation results of the 3-D model were first compared with 1-D model in no

crosswind condition. Fig. 6 gives the comparison of the 1-D and 3-D model results in

different ambient temperature. The numerical results of the 3D model in no-wind case have a

good consistence with the 1-D model. The deviations between the 1-D and 3-D models in air

outlet mass flow rate and air outlet temperature are about 1.3% and 1.5% respectively.

Figure 6 Comparison between the 1-D model and the 3-D model: ■ ma calculated by 1-D

model □ ma calculated by 1-D model ● Tao calculated by 1-D model ○ Tao calculated by 3-D

model

Different crosswind velocities ranging from 0 m/s to 15 m/s are discussed in this paper.

The heat exchanger temperature is set to be 50 ℃ and the ambient temperature is 25 ℃ in the

simulation. Fig. 7 presents the air stream line of the cooling tower at different crosswind

velocities. In the simulation, the crosswind is aligned with z axis direction.

25 30 35 40 4550

60

70

80

90

100

110

Tao

(oC

)

ma (

kg

/s)

Ambient temperature (oC)

45

50

55

60

Figure 7: 3-D steam lines of the cooling tower in different crosswind condition

In Fig. 7 the ambient air is moving through the heat exchanger from the tower bottom

towards the top driven by the natural draft when there is no crosswind. In such a case, the air

flow near the edge of the heat exchanger is slightly smaller than that in the centre. This is

because the resistance near edge of the heat exchanger is larger than that in the centre area.

When crosswind appears, the air flow inside the tower is disturbed. Under low crosswind

speed (less than 4 m/s) condition, the air flow in the windward part of the heat exchanger is

decreased and thus smaller than that in the leeward part. Once the speed reaches 4 m/s or

above, the vortices are formed in the tower. The vortices at the bottom of the cooling tower

redistribute the hot air above the heat exchanger and thus further impair the heat transfer.

With the further increase of the crosswind speed, vortices are also generated at the top of the

cooling tower which making the air difficult to exit the tower. Consequently, both the air

flow process and the heat transfer in the tower are reduced by the crosswind.

Fig. 8 is the pressure profile at the bottom of the cooling tower. With the acceleration of

the air as it flows around the lower part of the cooling tower, a low pressure area is generated

at the windward part of the cooling tower and the air flow is seriously affected in this area.

When crosswind speed is small, this low pressure area decreases the air flow mass flow rate

in this area by decreasing the air pressure difference. As the crosswind increases, the pressure

distribution is even more un-uniform in windward area, which result in the sucking out of the

hot air from the cooling tower. Thus in Fig.7, the vortices are seen in this area when the

crosswind speed is large enough.

Figure 8 Pressure contour at the bottom of the heat exchanger at crosswind speed of 0 m/s

and 4 m/s

Fig. 9 presents the temperature contours at the middle cross section of the tower. With the

influence of the crosswind, a high temperature region was formed on the windward side of

the cooling tower, due to the decreasing of the amount of cold air in this area. On the other

hand, for the cooling towers which have the horizontally placed heat exchanger, the heat can

be dumped from both the bottom and the top of the tower [18]. According to the temperature

contour, when crosswind exists, the heat from the heat exchanger will be taken away by both

the upward air steam created by the buoyance force and the forced convection created by the

wind. Thus in Fig. 9, a high temperature area is generated at the bottom of the heat exchanger

when the crosswind occurs.

Figure 9: The temperature contour of the tower in different crosswind condition

Figure 10 The heat transfer rate of the heat exchanger when crosswind speed is 0 m/s, 3 m/s,

8 m/s and 10 m/s

Fig. 10 shows the heat flux of the 18 heat exchangers at the bottom of the cooling tower

under different crosswind conditions. The heat flux in each heat exchanger bundles was

almost the same when there is no wind. When the crosswind speed is 3 m/s, the low pressure

effect showed in Fig.8 is produced at the windward part of the heat exchanger. This effect

decreases the upward air flow thus reduce the cooling performance in this area. However,

with the appearance of the crosswind, the force convection occurs at the bottom of the

cooling tower. According to previous research [18, 19], the heat transfer coefficient of the

force convection of a plane can be calculated by , where Rex is based on the

crosswind speed and the length of the heat exchanger. So with the increase of the crosswind

speed, the amount of the heat transferred by the force convection increased. So in Fig. 9, the

heat exchanger performance at the windward part is improved gradually. The crosswind

decreases the buoyance effect by disturbing the air flow and the heat transfer in the tower.

But it also increases the force convection at the bottom of the heat exchanger.

Figure 11 Heat rejection ratio and air mass flow rate at different crosswind condition: ■, t j ct on

t , ●, m ss flow t

Fig. 11 demonstrates the variation of the air mass flow rate and heat rejection rate under the

crosswind speed range from 0 m/s to 15 m/s. When the crosswind increasing from 0 m/s to 6

m/s, the heat rejection rate decreases rapidly with the increase of the crosswind velocity, due

to the adversely crosswind effect on the air flow mass flow rate. When vcw=6 m/s, the air

mass flow rate reached the minimum as well as the heat rejection rate. Fig. 12 presents the air

mass flow rate distribution under crosswind condition of 0 m/s and 6 m/s, the number of each

heat exchanger can be found in Fig.8. It can be seen that the air mass flow rate of the heat

exchanger bundles located at the windward direction of the cooling tower decrease more than

70% compared with the windless condition. These air-cooled heat exchangers do not function

well because of the supply of the cold air is too small. However, when vcw exceed 6 m/s, the

air mass flow rate varies little and the heat rejection rate increases with the increase in the

crosswind velocity. This is because of the forced convection at the bottom of the heat

exchangers. It can be expected from the curve that if the crosswind speed is sufficiently high,

the total heat dumping rate of the cooling tower will recover to the level in non-wind case.

However, the crosswind is still a negative factor on the cooling performance since the natural

draft effect is the main source to dump the heat in a NDDCT.

0 2 4 6 8 10 12 14

1000

1200

1400

1600

1800

2000

ma(k

g/s

)

Q(k

W)

vcw

(m/s)

0 2 4 6 8 10 12 14

50

100

150

200

Figure 12 Air mass flow rate of each heat exchanger bundle when crosswind is 0 m/s and 6

m/s

The above simulation results provide technical data and guidelines to the future

experimental work. The simulations predict the possible air and water temperature ranges

during the operation of the full scale experimental cooling tower. The ranges help to select

proper instrumentations and refine the testing procedure, so that the experiment cost can be

minimized. On the other hand, the planed future experiment work will be used to validate the

simulation results. The performance of the cooling tower will first test at the windless

condition and in different ambient temperature under which the 1-D model simulation was

carried out. Then the crosswind affect will be considered, the 3D CFD results of the

temperature distribution and air velocity distribution in the cooling tower and the thermal

performance of each heat exchanger bundle will be compared to the measurement.

5. CONCLUSIONS

The full scale 1-D and 3-D model of the Gatton NDDCT has been established. The

influence of hot water inlet temperatures, ambient temperatures and crosswind velocities on

the cooling performance a 20m NDDCT was analysed and discussed in this paper. The

current study draws the following conclusions:

1. Under non-crosswind condition, the heat rejection rate of the current cooling tower is able

to reject about 1.9MW to 3.1MW at the inlet water temperature of 70℃ when the ambient

temperature ranges in 15℃ to 35℃. This tower is promising for a 2~3MW CST plant using

supercritical CO2 cycle proposed by the University of Queensland.

N1 N2 N3 N4 N5 N6 N7 N8 N9 N10N11N12N13N14N15N16N17N180

2

4

6

8

10

ma (

kg

/s)

Heat exchanger bundle number

vcw

=6 m/s

vcw

=0 m/s

2. Both the inlet water temperature and the ambient temperature have significant influence on

the cooling performance of the NDDCT. These two factors influence the tower performance

through decrease the driving force of the air flow and heat transfer process.

3. The heat can be dumped from both the bottom and the top in a short NDDCT when the

crosswind exists. The crosswind decreases the buoyance effect by disturbing the air flow and

the heat transfer in the tower, but it causes the forced convection at the bottom of the tower as

well.

ACKNOWLEDGEMENTS

This research was performed as part of the Australian Solar Thermal Research Initiative

(ASTRI), a project supported by Australian Government. The author, Xiaoxiao Li, would

also like to thank China Scholarship Council (CSC) for their financial support.

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